Process for producing stabilized anatase titanium dioxide surfaces for durable adhesive bonding

ABSTRACT

The conventional Phosphate-Fluoride process for treating titanium parts to produce anatase titanium dioxide surfaces for adhesive bonding, which includes (1) an acid pickling bath step, (2) a phosphate-fluoride bath step, and (3) a warm bath step, is modified by adding about 2%, by weight, of an alkali metal sulfate, such as a neutral sulfate of sodium or lithium, to the bath in any of steps (1), (2) and (3), to stabilize the anatase surface and thereby prevent transformation thereof to the undesirable rutile structure.

United States Patent [191 Hamilton et a1.

[ Dec. 23, 1975 PROCESS FOR PRODUCING STABILIZED ANATASE TITANIUMDIOXIDE SURFACES FOR DURABLE ADHESIVE BONDING [75] Inventors: Willard C.Hamilton, Webster,

N.Y.; Raymond F. Wegman, Ledgewood, NJ.

[73] Assignee: The United States of America as represented by theSecretary of the Army, Washington, DC.

22 Filed: Jan. 4, 1974 21 Appl. No.: 430,825

[52] US. Cl. 156/319; l48/6.l4 R; 148/6.l5 R; 148/6.17

[51] Int. Cl. C23F 7/06; CO9J 5/04 [58] Field of Search 148/6.14 R, 6.15R, 6.17; 117/127, 49; 156/319 [56] References Cited UNITED STATESPATENTS 2,864,732 12/1958 Miller et a1. l48/6.l5 R 3,041,215 6/1962Jenkins et al. 148/6.l4 R

FOREIGN PATENTS OR APPLICATIONS 729,148 3/1966 Canada l48/6.14

3/1967 Canada 1. 148/6.14

OTHER PUBLICATIONS Sastry, R.L.N., Effect of Some Impurities on theAnatase-Rutile Transformation, Indian Journal of Chemistry, Vol. 3,Sept. 1965, pp. 414-415.

Primary Examiner-Ralph S. Kendall Assistant ExaminerCharles R. Wolfe,Jr.

Attorney, Agent, or FirmNathan Edelberg; Robert P. Gibson; Thomas R.Webb [57] ABSTRACT The conventional Phosphate-Fluoride process fortreating titanium parts to produce anatase titanium dioxide surfaces foradhesive bonding, which includes 9 Claims, N0 Drawings PROCESS FORPRODUCING STABILIZED ANATASE TITANIUM DIOXIDE SURFACES FOR DURABLEADHESIVE BONDING BACKGROUND AND SUMMARY OF THE INVENTION The presentinvention relates to the treatment of titanium parts, either puretitanium or alloys of titanium with other metals such as aluminum, toproduce anatase titanium dioxide surfaces that can be bonded to otherparts, e.g. by an epoxy type adhesive, to produce durable, long-lifebonds.

An early method of preparing titanium surfaces for bonding, used in theproduction of UH-l aircraft by Bell Helicopter Company in 1963, involvedcleaning the metal with an alkaline cleaner. This method produced asurface containing the rutile structure of titanium dioxide. Althoughthis surface will accept the adhesive and produce very good initial bondstrengths, it will not produce a satisfactory durable bond. The surfacehas been found to have a preference for moisture, rather than theadhesive, and when exposed to a moist atmosphere will become unbondedfrom the adhesive.

In 1966, a Phosphate-Fluoride process for treating titanium wasintroduced by Bell Helicopter Company in the manufacture of all of itsaircraft. This process includes the following steps:

A. Immersing clean titanium parts for several minutes at roomtemperature in an aqueous pickling solution containing hydrofluoric andnitric acids;

B. Soaking the parts for several minutes at room temperature in anaqueous solution containing hydrofluoric acid, trisodium phosphate andpotassium fluoride;

C. Soaking the parts in hot water for at least minutes; and

D. Various rinsing and drying steps.

In accelerated tests performed on epoxy-bonded joints incorporatingtitanium parts processed by the above methods, we have found that thechange in processing from the alkaline method to the Phosphate- Fluorideprocess resulted in an increase in the durability of the joints fromabout 10 hours for the alkaline method to about 6575 hours for thePhosphate-Fluoride method. These accelerated tests involved subjectingthe bonded joint to a sustained load of 880 psi (pounds per square inch)at 60C. and 95% relative humidity. We also found that the surfaceproduced by the PhosphateFluoride process was the anatase structure oftitanium dioxide, and that the anatase structure has a preference forthe adhesive rather than water. However, we also found that this anatasestructure undergoes a change to the rutile structure when the joint issubjected to elevated temperature, high humidity and stress. This changein structure has been shown to occur in test specimens and in aircraftcomponents returned from service after bond failures were found. Thecrystalline rearrangement is accompanied by about an 8% change involume.

Based upon the above findings of crystalline structure and rearrangementwe embarked on a program of investigation, the intent of which was toproduce a stable surface layer containing the anatase structure oftitanium dioxide. The surface layers had to be produced in thin layersby a practical production method without affecting the properties of thetitanium or titanium alloy itself. I

Investigations made, at the Indian Institute for Science, and elsewhere,on the effects of the presence of various impurities on thetransformation of the anatase to the rutile structure of titaniumdioxide indicated that certain impurities tend to inhibit thistransformation. For example, in a paper entitled Effect on SomeImpurities on the Anatase-Rutile Transformation, by R. L. N. Sastry,Indian Journal of Chemistry, Vol. 3, September, 1965, pp. 414415, theauthor reported that the alkali metal ions, ammonium, barium, sodium andpotassium hinder the transformation, whereas the lithium ion favors thetransformation. He stated that both S0 and Na (ions) inhibit thetransformation, and observed that the effect was additive in the case ofsodium sulphate. Some of the various investigations of theanatase-rutile transformation related to titanium dioxide used as awhitener in paints. We know of no suggestion for purposely addingcertain impurities to processes for preparing titanium for adhesivebonding to improve the durability of the resulting bonds.

An object of the present invention is to provide stable anatase titaniumdioxide surfaces on titanium parts for durable adhesive bonding to otherparts.

In accordance with the present invention, the Phosphate-Fluoride processfor treating titanium parts to produce anatase titanium dioxide surfacesfor adhesive bonding is modified by adding about 2%, by weight, of aneutral alkali metal sulfate to the solution or water in any of (1) theacid pickle step, (2) the phosphate-fluoride soak step, or (3) the warmwater soak step, to stabilize the anatase structure and thus preventtransformation thereof to the undesired rutile structure. Acceleratedtests, under the conditions given above, on bonded joints made withtitanium dioxide surfaces prepared by such modifications of thePhosphate-Fluoride process have shown that the addition of the alkalimetal sulfate in the basic process increased the durability of thejoint, at 880 psi., from about 6575 hours, by 480 to 700%. The highestdurability, 528 hours average, was produced by adding 2%, by weight, ofsodium sulfate to the acid pickle step (I), while the lowest durability,361 hours average, was produced by adding 2%, by weight, of lithiumsulfate to the phosphate-fluoride step (3).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The complete flowchart of the conventional Phosphate-Fluoride process for treatingtitanium parts is as follows:

Step 1. Rinse the parts with a solvent, e.g. acetone, at

room temperature.

Step 2. Immerse in an alkaline cleaner, e.g. aqueous Oakite HD 126, for5 to 15 minutes at 60 to 80C.

Step 3. Rinse with running water at room temperature to 80C.

Step 4. Immerse for about 2 minutes at room temperature in an aqueouspickle solution containing 1.8%, by volume, of HF, 35%, by volume, of70% HNO and the rest water.

Step 5. Rinse with water at room temperature.

Step 6. Soak for about 2 minutes at room temperature in an aqueoussolution containing 2%, by volume, of 70% HF, 5.1%, by volume, of Na PO2.1%, by weight, of KF, and the rest water.

Step 7. Rinse with running water at room temperature.

Step 8. Soak in water for about 15 minutes at about Step 9. Rinse withrunning water at room temperature.

Step 10. Dry with air at room temperature to 65C. Deionized water isused in steps 2 through 9.

Average durabilities of 75, 25 and 9 hours were obtained from theaccelerated tests referred to above on epoxy-bonded joints includingtitanium surfaces treated by this conventional Phosphate-Fluorideprocess (without modification), at stresses of 880, 1,320 and 1,760lbs./in. respectively.

In accordance with the invention, any of steps 4, 6 and 8 of theconventional Phosphate-Fluoride process is modified by the addition ofseveral ounces per gallon of a neutral alkali metal sulfate to the bath,to stabilize the anatase structure of the titanium dioxide surface. Fourexamples of modified processes which were carried out and testedsuccessfully will be described. In each example, the titanium parts usedwere Titanium 6, 4 Alloy, which consists of 6% aluminum, 4% vanadium,and the remainder titanium.

EXAMPLE I Step 1. Degrease, with vapor or solvent; Step 2. Alkalineclean: immerse parts in alkaline cleanser for -15 minutes at 60 to 80C.;Step 3. Rinse, with water at room temperature to 80C.; Step 4. Pickle:immerse for 2 minutes at room temperature in an aqueous solution of:1.8%, by volume, of 70% hydrofluoric acid, 2%, by weight, of neutralsodium sulfate, and 35%, by volume, of 70% nitric acid, and the restwater; Step 5. Rinse, with water at room temperature; Step 6. PhosphateFluoride Treat: soak parts for 1.5 to 2.5 minutes at room temperature inan aqueous solution of: 5.1%, by volume, trisodium phosphate, of 2.1%,by weight, of potassium fluoride, 2%, by volume, of 70% hydrofluoricacid, and the rest water; Step 7. Rinse, with water at room temperature;Step 8. Hot Water Soak: use water at about 65C. for

minutes; Step 9. Final Rinse, with water at room temperature for 30seconds to 1 minute; and Step 10. Dry; with air at room temperature at65C. Preferably, deionized water is used in steps 2 through 9. Theprocess of Example I differed from the conventional Phosphate-Fluorideprocess by adding 2%, by volume, of neutral sodium sulfate in picklingstep 4.

Average durabilities of 528, 105 and 61 hours, were obtained fromaccelerated tests, at stresses of 880, 1,320 and 1,760 lbs./in.respectively, conducted under the conditions described above, on manysamples of adhesive bonds made with titanium parts treated by theprocess of Example I. It should be noted that the improvement indurability produced by the new titanium treatment of Example I over theconvention treatment was about 700%, 420% and 680%, at 880, 1,320 and1,760 lbs./in. respectively.

EXAMPLE II In Example 11, the conventional Phosphate-Fluoride processwas modified solely by adding 2%, by weight, of neutral lithium sulfateto the solution in step 6, and phosphaste-fluoride soak step.

EXAMPLE III In Example 111, the conventional Phosphate-Fluoride processwas modified solely by adding 2%, by weight, of neutral sodium sulfateto the hot water bath in step 8.

EXAMPLE IV In Example IV, the conventional Phosphate-Fluoride processwas modified by:

a. adding 2%, by weight, of neutral sodium sulfate to the picklingsolution in step 4; and b. adding 2%, by weight, of neutral lithiumsulfate to the phosphate-fluoride solution in step 6. Averagedurabilities obtained from accelerated tests, as described above, onsamples of adhesive bonds made with titanium parts treated by theprocesses of Examples II, III and IV, respectively, are listed incolumns C, D and E of the following table, wherein column A contains thedurabilities for the conventional process and column B contains thedurabilities for Example 1:

The table shows that the durabilities for Examples 11 and III are higherthan for Example I at 1,320 1bs./in. but lower at 880 and 1,760 lbs./in.Column E shows that the addition of the neutral alkali metal sulfate totwo of the steps in the process of Example IV does not produce anygreater effect than the addition thereof to only one step, as in ExampleI, II and 111. Each of the durabilities listed in the above table wasobtained by averaging anywhere from 5 to 16 different tests made underthe same conditions, and the individual values for these tests variedconsiderably, largely due to the failure of some bonds for reasons otherthan a change from anatase to rutile structure, which accounts for someof the variations in the results of various processes. For example, thevalue 14 for Example 111 (column D) at 1,760 lbs./in. was obtained byaveraging 18, 40, 37, 3, 3, 4, 4 and 2. If the last five values areignored as failures resulting from some other cause, the average valuebecomes 27, which is close to the corresponding values for Examples 11and IV (C and E), and represents a minimum of about 300% improvement indurability produced by the present invention at 1,760 lbs./in.

We claim:

1. In the method of adhesive bonding of titanium parts to other partswherein said titanium parts are subjected, prior to adhesive bonding, toa phosphatefluoride process to produce anatase titanium dioxide surfacesfor adhesive bonding, which process includes (1) an acid pickle bathstep, (2) a phosphate-fluoride bath step, and (3) a hot water bath step,and said anatase titanium dioxide surfaces, after being produced, areadhesively bonded to other parts,

the improvement consisting of adding about 2% of a neutral alkali metalsulfate to the bath in any of said three steps for stabilizing theanatase surfaces and thereby preventing transformation thereof to theundesired rutile structure.

2. The method as in claim 1, wherein said phosphatefluoride processcomprises the following steps, in the order named:

A. cleaning said parts;

B. rinsing said parts with water at room temperature to 80C.;

C. immersing said parts, for about 2 minutes at room temperature, in anaqueous pickling solution containing: about 2%, by volume, 70% HF, about35%, by volume, 70% HNO and the rest water;

D. rinsing said parts with water at room temperature; E. soaking saidparts, for about 2 minutes at room temperature, in an aqueous solutioncontaining:

about 2%, by volume, 70% HF, about 5%, by weight, Na PO about 2%, byweight, KF, and the rest water; F. rinsing said parts with water at roomtemperature;

Nagso 6 G. soaking said parts in water, for about 15 minutes at about65C.; H. rinsing said parts with water at room temperature for about 45seconds; and

l. drying said parts in air at about 65C; I and any of the steps C, Eand G is modified by adding to the bath about 2%, by weight, of asulfate of sodium lithium, for stabilizing said anatase surfaces asstated.

3. The method as in claim 2, wherein deionized water is used in each ofsteps B through H.

4. The method as in claim 2, wherein the sulfate is added to thepickling solution in step C.

5. The process as in claim 4, wherein the sulfate is Na SO 6. The methodas in claim 2, wherein the sulfate is added to the solution in step E.

7. The method as in claim 6, wherein the sulfate is Li SO 8. The methodas in claim 2, wherein the sulfate is added to the water in step G.

9. The method as in claim 8, wherein the sulfate is

1. IN THE METHOD OF ADHESIVE BONDING OF TITANIUM PARTS TO OTHER PARTSWHEREIN SAID TITANIUM PARTS ARE SUBJECTED, PRIOR TO ADHESIVE BONDING, TOA PHOSPHATE-FLUORIDE PROCESS TO PRODUCE ANATASE TITANIUM DIOXIDESURFACES FOR ADHESIVE BONDING, WHICH PROCESS INCLUDES (1) AND ACIDPICKLE BATH STEP, (2) A PHOSPHATEFLUORIDE BATH STEP, AND (3) A HOT WATERBATH STEP, AND SAID ANATASE TITNIUM DIOXIDE SURFACES, AFTER BEINGPRODUCED, ARE ADHESIVELY BONDED TO OTHER PARTS, THE IMPROVEMENTCONSISTING OF ADDING ABOUT 2% OF A NEUTRAL ALKALI METAL SULFATE TO THEBATH IN ANY OF SAID THREE STEPS FOR STABILIZING THE ANATASE SURFACES ANDTHEREBY PREVENTING TRANSFORMATION THEREOF TO THE UNDESIRED RUTILESTRUCTURE.
 2. The method as in claim 1, wherein said phosphate-fluorideprocess comprises the following steps, in the order named: A. cleaningsaid parts; B. rinsing said parts with water at room temperature to80*C.; C. immersing said parts, for about 2 minutes at room temperature,in an aqueous pickling solution containing: about 2%, by volume, 70% HF,about 35%, by volume, 70% HNO3, and the rest water; D. rinsing saidparts with water at room temperature; E. soaking said parts, for about 2minutes at room temperature, in an aqueous solution containing: about2%, by volume, 70% HF, about 5%, by weight, Na3PO4, about 2%, by weight,KF, and the rest water; F. rinsing said parts with water at roomtemperature; G. soaking said parts in wAter, for about 15 minutes atabout 65*C.; H. rinsing said parts with water at room temperature forabout 45 seconds; and I. drying said parts in air at about 65*C; and anyof the steps C, E and G is modified by adding to the bath about 2%, byweight, of a sulfate of sodium lithium, for stabilizing said anatasesurfaces as stated.
 3. The method as in claim 2, wherein deionized wateris used in each of steps B through H.
 4. The method as in claim 2,wherein the sulfate is added to the pickling solution in step C.
 5. Theprocess as in claim 4, wherein the sulfate is Na2SO4.
 6. The method asin claim 2, wherein the sulfate is added to the solution in step E. 7.The method as in claim 6, wherein the sulfate is Li2SO4.
 8. The methodas in claim 2, wherein the sulfate is added to the water in step G. 9.The method as in claim 8, wherein the sulfate is Na2SO4.